This invention relates to an aqueous newsprint ink and to an improved process for printing newsprint paper with such ink.
A typical newspaper printing press in current use is a letterpress unit comprised of a train of rollers contiguously arranged to distribute a thin film of newsprint ink for transfer to the typographic or photopolymer plate. The letterpress system presently in use is a heavy piece of equipment and necessarily consumes large amounts of energy, a major factor in determining the cost of the installation and operation. Moreover, newspaper printing by the letterpress technique requires frequent adjustments of roller settings to maintain a uniform ink application, thereby necessitating additional manpower to monitor the system. At times, letterpress printing also creates ink misting, whereby at high speeds an ink which is too thin will cause a fine spray of ink to be thrown off by the rapidly moving rollers, thereby coating the press and its environs with a sticky layer of ink. Ink misting may be attributed to an improperly formulated ink composition, hard or imperfect rollers, old or uneven blankets or improper roller settings.
Many of these equipment difficulties have been overcome by the use of a recently introduced simplified light-weight printing system which contains fewer rollers, commonly called a "keyless" system because it lacks the adjusting keys used to control the ink on the rollers of standard letterpress equipment. One example of this light-weight equipment is the ANPAPRESS (trademark) system developed by the American Newspaper Publishers Association Research Institute. The keyless letterpress has many desirable characteristics, including reduced ink misting and paper waste, as well as decreased space and energy requirements due to the light-weight design.
The standard newsprint inks employed almost exclusively in both the letterpress and keyless systems consist of printing-grade carbon black pigment (or dyes and/or other pigments to accentuate color density or for colored ink) suspended in mineral oil in the optional presence of an oil-soluble toner dye such as induline base for high-speed presses or a small amount of rosin oil or other resin for slower-moving presses. Mineral oil is used as the pigment vehicle because it remains fluid until it penetrates into the absorbent newspaper stock. In contrast, comparable newsprint inks containing water and/or organic solvents evaporate before they reach the stock, and are thus generally considered undesirable for use in both the letterpress and keyless printing systems.
The standard newsprint ink, however, suffers from a number of disadvantages. One is that the mineral oil vehicle contained therein is a petroleum derivative, with its accompanying high cost, pollution effects and dependence for its supply on fluctuating markets. In addition, mineral oil acts only as a carrier for the pigment, not a binder. When the ink is deposited, most of the mineral oil penetrates into the porous stock; but due to the nature of mineral oil, the ink that remains on the surface never dries. As a result, "offsetting" occurs whereby part of the printed image is transferred to another surface which has come into contact with the ink, e.g., the ink is readily removed by rubbing the surface. The oil may also penetrate the stock and show as an oil stain or "strike through" on the back of the sheet, imparting a brownish tone to the paper. Moreover, it is difficult to obtain a clear printed copy with standard newsprint inks because the mineral oil, as it is absorbed, creates a halo around the printed image, thereby blurring the edges of the image.
Accordingly, it is an object of the present invention to provide an improved newsprint ink derived from readily available, less expensive raw materials with fewer accompanying pollution problems, which ink contains a binder to fix the pigment into the paper so that "striking through", "offsetting" and blurring of the printed image are minimized or eliminated.
It is another object to provide a newsprint ink especially designed for use in conjunction with the light-weight keyless printing press system. It is a further object to provide an improved printing process whereby an aqueous newsprint ink is employed.